1. Field of the Invention
The present invention relates to sensors for sensing gases, and, more particularly, to sensors for sensing gaseous leaks from valves, flanges, fittings, and the like, in chemical and petroleum plant (or refinery) environments.
2. Description of Related Art
Gaseous emissions are of two types: (a) stack gas emissions, and (b) fugitive emissions (i.e., leaks), which are about half of the total emissions. Major chemical companies say they will reduce their emissions by about 90% over the next 5 years. It will be impossible for them to comply without reducing fugitive emissions.
Most of the industrial air pollutant emissions are generated by two types of companies, chemical manufacturing companies and oil refineries, in which there is a huge potential for fugitive emissions. Both types of companies have processing facilities that are characterized by a large number of valves, flanges, and fittings, each of which is a potential source of fugitive emissions. A typical facility will have about 40,000 valves and about four to six times as many flanges, or about 100,000 to 300,000 of these potential leak sources.
Currently, personnel with sniffers walk through the facility periodically and check for leaks. If there is a continuous problem with a valve or fitting, it is bagged and samples are taken for analysis by gas chromatography. High leakers, defined as those with 50,000 ppm or more of hydrocarbons in the surrounding atmosphere, must be identified quickly and repaired within 24 hours. A leaker is defined as a fitting with 500 ppm or more hydrocarbon in the surrounding air, although some companies use a lower value.
The hand-held sniffer typically contains a flame ionization detector. Some fittings are checked as infrequently as once or twice a year. If a fitting leaks, the leak will continue until the next check period. These leaks also present an explosion hazard if they go undetected.
The usual sniffer consists of a long tube through which air samples are sucked to reach the flame ionization detector. There is often difficulty in reaching some valves, such as those near the ceiling, or those on a offshore rig, for sampling. The current maintenance program is costing the petroleum industry about $1 to $4 per year per component for leak detection.
There is also a big problem with data management. Each fitting must be recorded when it is checked. Systems with leaking valves must also be taken off line to make replacements, and one must be sure that the correct valve is being replaced, because all this is very costly.
A critical, large, and currently unmet need is the ability to place inexpensive monitors at a significant number of these sites, and to do this easily and flexibly. Readings from these sensors would be monitored and recorded by a computer, which would also notify the operators immediately when and where a leak has occurred. The detection of leaks on a daily basis would enable the operators to fix the leaks in a more timely fashion, so that the overall quantity of fugitive emissions would be greatly reduced.
The specificity of the sensor can be very low, since the operators of the plant know what is in the system. Sources of ignition, such as flames, hot wires, or hot catalytic surfaces, which are present in the sensors that are currently available, must be avoided because of explosion hazards. The sensitivity of the sensors can be quite low, since they can be placed near the source of the leak, or the leak can be partially confined in the region of the sensor, and the sensor can be placed in the confined region where the vapor concentration is very high. A sensor chip would be ideal.